As is known, an electro-optic element having a plurality of individually addressable electrodes may be employed as a multigate light valve in, say, an electro-optic line printer. See a copending and commonly assigned United States patent application of R. A. Sprague et al., which was filed June 21, 1979 under Ser. No. 040,607 now U.S. Pat. No. 4,281,904 on a "TIR Electro-Optic Modulator with Individually Addressable Electrodes". Also see, "Light Gates Give Data Recorder Improved Hardcopy Resolution," Electronic Design, July 19, 1979, pp. 31-32; "Polarizing Filters Plot Analog Waveforms,"Machine Design, Vol. 51, No. 17, July 26, 1979, p. 62; and "Data Recorder Eliminates Problem of Linearity,"Design News, Feb. 4, 1980, pp. 56-57.
Substantial progress has been made in developing such light valves and in applying them to electro-optic line printing. For example, a copending and commonly assigned United States patent application of R. A. Sprague, which was filed Sept. 17, 1980 under Ser. No. 187,911 now U.S. Pat. No. 4,389,659 on an "Electro-Optic Line Printer," shows than an image represented by a serial input data stream may be printed on a standard photosensitive recording medium through the use of a multigate light valve that is illuminated by a more or less conventional light source. That disclosure is of interest primarily because it teaches input data sample and hold techniques for minimizing the output power required of the light source. Another copending and commonly assigned United States patent application of W. D. Turner, which was filed Sept. 17, 1980 under Ser. No. 187,936 on "Proximity Coupled Electro-Optic Devices," reveals that the electrodes and the electo-optic element of a multigate light valve may be physically distinct components which are pressed or otherwise firmly held together to achieve "proximity coupling." Still another copending and commonly assigned United States patent application of R. A. Sprague et al., which was filed Sept. 17, 1980 under Ser. No. 188,171 now U.S. Pat. No. 4,367,925 on "Integrated Electronics for Proximity Coupled Electro-Optic Devices," shows that it is relatively easy to make the necessary electrical connections to the many electrodes of a typical proximity coupled multigate light valve if the electrodes are formed by suitably patterning a metallization layer of, say, a VLSI silicon electrode driver circuit. Furthermore, yet another copending and commonly assigned United States patent application of W. D. Turner et al., which was filed Sept. 17, 1980 under Ser. No. 187,916 on "Differential Encoding for Fringe Field Responsive Electro-Optic Line Printers," teaches that the number of electrodes required of a multigate light valve to enable an electro-optic line printer to achieve a given resolution is reduced by a factor of two if the input data is differentially encoded.
For successful line printing with a multigate light valve of the foregoing type, the zero order diffracted components of the light emerging therefrom are brought to focus at the center of the entrance pupil of an imaging lens (hereinafter simply referred to as the center of the imaging lens) which, in turn, images the light valve onto the recording medium. In other words, the zero order components of the output beam are collected as the principal rays for imaging. This ensures that substantially the same amount of light is collected from each of the many electrode pairs of the electro-optic element and that more or less uniform imaging conditions are maintained across essentially the full width of the light valve.
Prior multigate light valves have generally had parallel electrode geometries. For that reason, the standard practice has been to illuminate such a light valve with an input beam which is collimated in a direction parallel to the electrodes, such that the zero order diffracted components of the output beam emerge along parallel paths which are normal to the light valve. This means tha a telecentric imaging system is needed to collect the zero order diffraction components as the principal rays for imaging (as a matter of definition, a "telecentric imaging system" is one in which all principal rays are normal to the surface of the object being imaged). For example, a field lens is commonly used to bring the zero order light to focus at the center of an imaging lens. Unfortunately, however, the field lens tends to create unwanted field curvature and to introduce undesirable optical abberations. Accordingly, a relatively complex and expensive imaging lens is usually required to obtain a reasonably flat and distortion free image of the light valve in the image plane (i.e., on the suface of the phosensitive recording medium).